HVAC systems can be used to regulate the environment within an enclosure. Typically, an air blower is used to pull air from the enclosure into the HVAC system through ducts and push the air back into the enclosure through additional ducts after conditioning the air (e.g., heating or cooling the air). For example, a furnace, such as a gas furnace may be used to heat the air.
High efficiency residential gas-fired appliances typically rely on a mechanical means, such as a combustion air inducer, to create controlled mass flow thru the flue side of the appliance heat exchanger. For flow to occur, a pressure differential must exist across the heat train of the furnace. To verify that the proper pressure drop and flow are achieved and maintained to support the combustion process of the furnace within safe limits, pressure sensing devices are typically employed. These pressure sensing devices might include mechanical differential pressure switches or electronic sensors which provide feedback to an integrated electronic control.
The components of a furnace which create, control and sense flow thru the heat exchangers are typically designed to be employed on multiple size furnaces (low to high input). Accordingly, the various components are sized to handle the range of inputs. This type of furnace design is similar among industry manufacturers, and typically employs a flue gas/condensate collector box attached to the end of the condenser coil (referred to herein as a Cold End Header Box (CEHB)), a combustion air inducer fan assembly (CAI), a fixed orifice located in the CAI or CEHB to regulate flow through the heat train, and a pressure sensing device to monitor flow. The pressure sensing device could be used to monitor pressure across the fixed orifice, or other points in the heat train to provide the most advantageous signal for the application.